Although so-called radiography employing silver halide photography is conventionally used to obtain radiographic images, there also has been developed a radiographic imaging method not using silver halide photographic material. Thus, an imaging method is disclosed, in which radiation that has been transmitted through an object is absorbed by phosphor, followed by exciting the phosphor with an energy to cause a radiation energy accumulated in the phosphor to radiate in the form of fluorescence, and imaging is achieved by detecting the fluorescence.
Specifically, U.S. Pat. No. 3,859,527 discloses a radiation image conversion method, in which a panel comprising on a support a photostimulable phosphor layer is employed using either or both visible light and infrared rays as the stimulating energy. There have been developed radiation image conversion methods using a photostimulable phosphor(hereinafter, also denoted simply as a stimulable phosphor) exhibiting enhanced luminance and high sensitivity, including, for example, a radiation image conversion method employing BaFX:Eu2+ type phosphor (X:Cl, Br, I), as described in JP-A No. 59-75200 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication); a radiation image conversion method employing an alkali halide phosphor, as described in JP-A No. 61-72087; a radiation image conversion method employing an alkali halide phosphor containing, as co-activators, Tl+and metals such as Ce3+, Sm3+, Eu3+, Y3+, Ag+, Mg2+, Pb2+or In3+, as described in JP-A Nos. 61-73786 and 61-73787.
Recently, a radiation image conversion panel exhibiting further enhanced sharpness has been desired in the field of diagnostic image analysis. Of these, an attempt in controlling the form of stimulable phosphor grains to enhance sensitivity and sharpness was made as a means for improving sharpness of radiographic images. For example, JP-A No. 61-142497 discloses a method-of using a stimulable phosphor layer comprising a fine columnar block which has been formed by sedimentation of a stimulable phosphor on a support having fine protruded patterns; JP-A 62-39737 discloses-a method of using a radiation image conversion panel having a stimulable phosphor layer having a pseudo-columnar form which has been formed by producing cracks on the layer surface side; JP-A 62-110200 proposes a method in which a stimulable phosphor layer having voids is formed by vapor deposition onto the upper surface of a support, followed by growing voids by subjecting a heating treatment to produce cracks.
JP-A No. 2-58000 proposed a radiation image conversion panel having a stimulable phosphor layer, in which long and thin columnar crystals were formed with an incline at a given angle toward the direction normal to the support.
In the foregoing attempts to control the form of a stimulable phosphor layer, it was intended to enhance image quality by allowing the phosphor layer to have a columnar crystal structure. It was supposed that the columnar form prevented traverse diffusion of stimulated emission light (or photo-stimulated luminescence), i.e., the light reached the support surface with repeating reflection at the interface of cracks (or columnar crystals), thereby leading to markedly enhanced sharpness of images formed by the stimulated luminescence.
Recently, a radiation image conversion panel using a stimulable phosphor containing an alkali halide such as CsBr as a basic substance and Eu as an activator, and the use of activator Eu leading to enhanced X-ray conversion efficiency, which has formerly not been achieved. However, Eu exhibits a marked thermal diffusion and also exhibits relatively high vapor pressure under vacuum, producing problems that Eu often scatters or localizes in parent material, thereby making it difficult to achieve an intended high X-ray conversion efficiency, so that practical use has not been accomplished on the market.
However, it has been desired to form a phosphor layer of CsBr:Eu through vapor deposition system. In the formation of a deposit layer, a substrate and an evaporation source is precisely arranged to control the distribution of layer thickness and the physical position is designed to perform the precise layer thickness distribution. Specifically in CsBr:Eu, thermal diffusion of Eu is marked and the vapor pressure under vacuum is relatively high and scattering results in local existence of Eu in parent material, therefore, a deposition process and substrate material quality become important to achieve layer uniformity. Specifically, uniformity in close contact of a phosphor layer with the substrate is essential to realize enhanced performance as well as a large area and high thickness. When resin is placed on the substrate surface, remained solvent or volatile gradients resulting from the manufacturing process of the resin are present in a relatively large amount, producing volatile components during the phosphor layer formation, changing the degree of vacuum and resulting in variation in layer thickness.